Exemplary embodiments of the invention relate to a method for preparing to start a fuel cell system for a starting operation carried out in the event of switching off of the fuel cell system and/or in the event of the temperature falling short of a predetermined temperature threshold value, in order to discharge water and moisture from the fuel cell system.
Fuel cell systems are known to be used, for example, in vehicles to provide electrical drive power. One of the problems of fuel cell systems is the production of very pure water in the fuel cell system during operation. If temperatures fall below freezing point, which will inevitably happen in vehicle applications, the fuel cell system can freeze, which then causes corresponding problems in the event of a repeated starting operation or it requires a very long time until the fuel cell system can be started. This is because, due to moisture that has condensed out and frozen, for example gas ducts and/or valves can be blocked and functioning of other components can be impaired by the formation of ice.
A known solution to counteract this problem during the preparation of a fuel cell system for a starting operation, is to perform a start preparation routine when the fuel cell system is switched off in order to discharge water and moisture from the fuel cell system. The fuel cell system is typically flushed with gas, which is conveyed, for example, by the air conveying device and/or a hydrogen recirculation fan or another type of fan. In this way moisture is blown out of the fuel cell system and any water separators and the like can be emptied and flushed through in order also to discharge as much moisture as possible. During stoppage and the subsequent switch-off process fuel cell systems are typically still very hot, so that vapor potentially remains in the fuel cell system and can be condensed out later and, if the temperatures then fall below freezing point, can likewise lead to the problems described above. Therefore, in addition to or as an alternative to the start preparation routine described above it is also known to perform a start preparation routine when the fuel cell system is stopped. For this purpose, if, for example, the ambient temperature drops below a predetermined threshold value the fuel cell system is, for example, activated in order to then perform the start preparation routine and to dry the system. In contrast to drying immediately after the shutdown of the fuel cell system, this start preparation routine, which is frequently also designated as conditioning or downtime conditioning, also has the advantage that water that has condensed out can also be removed. Therefore, it is quite reasonable to carry out both routines.
According to the general prior art, the start preparation routine will typically always run following the same pattern, as a predetermined quantity of gas, for example air, is conveyed through the fuel cell system in order to discharge moisture and to blow water out. In order to achieve this, for example, all gas conveying equipment can be run at constant rotational speed for a predetermined time. In this case the quantity is typically always designed so that secure and reliable drying can be achieved in any case.
German patent document DE 10 2007 050 856 A1 discloses a method that performs the flushing in a very complex manner as a function of membrane hydration and average current density time profiles and the like. This is extraordinarily expensive and complex and can only be operated at considerable expense for detection of the required measured values, in particular for a start preparation routine that is carried out for downtime conditioning.
Moreover, the generic prior art, which includes German patent document DE 11 2007 000 300 T5, US patent document US 2006/222924 A1, and German patent document DE 10 2012 01947 A1, discloses a method in which the start preparation routine is adapted with reference to measured values which have been recorded during the operation of the fuel cell system.
Exemplary embodiment of the present invention are directed to a method for preparing a fuel cell system for a starting operation using a start preparation routine that functions very simply and efficiently and can be carried out very quickly, quietly and in an energy-efficient manner.
In the method according to the invention the start preparation routine is changed as a function of measured values in the fuel cell system. In this case these measured values are recorded within a predetermined time interval before the shutdown of the fuel cell system. Before the shutdown of the fuel cell system, when this is still in operation, measured values can be very simply recorded or are typically available in any case for specific control objectives and for influencing operating procedures in the fuel cell system. These measured values, which are in any case available, or the measured values which are simply to be detected, can then be used in order to adapt the system preparation routine accordingly. The crux of the idea behind it is that in a full load situation of the fuel cell system, for example in a time interval of approximately 15 minutes before the shutdown of the fuel cell system, much less moisture remains in the fuel cell system, since temperatures and volumetric flows are relatively high. Then a start preparation routine is dispensed with completely, or it is appropriately short or can be carried out at low rotational speeds of the gas conveying equipment for providing the air stream for drying the fuel cell system. This is energy-efficient, quick, and quiet. On the other hand, if the measured values recorded in the time interval before the shutdown of the fuel cell system indicate that a partial load situation or an extremely low load situation has occurred, then a very much more moist fuel cell system is to be expected, so that for careful drying of the fuel cell system the system preparation routine is carried out for a correspondingly longer time and/or at higher speeds of the gas conveying equipment. At least one intermediate stage exists therebetween. Thus because in the method the measured values are used that have been recorded within a predetermined time interval before the shutdown of the fuel cell system, an optimized system preparation routine, which is adapted in each case to the requirements of the fuel cell system with regard to energy requirement, noise emissions and time, can be used independently of a measurement during the system preparation routine itself.
In a very advantageous modification of the method according to the invention the measured values comprise cooling water temperatures in the fuel cell system. In addition to the mere evaluation of load requirements, in particular cooling water temperatures in the fuel cell system are values to be recorded very simply and efficiently and which are associated indirectly with the load requirements and directly with the situation with regard to the moisture that is present after the shutdown of the fuel cell system. Thus, a suitable start preparation routine can be selected very simply and efficiently for recording cooling water values.
In an advantageous modification thereof different cooling water temperature threshold values can be predetermined, wherein the measured values include the time for which the cooling water temperature was above the respective cooling water temperature threshold value. If, for example, the cooling water temperature within the last 15 minutes of the operation of the fuel cell system was 80% above an upper cooling water temperature threshold value of for example 60° C., then a very much shorter system preparation routine can be performed compared to when the cooling water temperature was only 10% above this upper cooling water temperature threshold value and in the rest of the time was above a lower cooling water temperature threshold value.
In a very favorable modification of the method according to the invention the measured values are assigned to individual classes according to the load situation and/or cooling water temperature of the fuel cell system within the time interval. A predetermined sequence of the system preparation routine is then assigned to each of the classes. Such an assignment to classes can take place immediately upon shutdown of the fuel cell system. Thus, for example, assignment to five different classes can take place, which symbolically represent full load, low load, low partial load, medium partial load and high partial load. Depending upon the load situation, a suitable system preparation routine can then be carried out upon shutdown of the fuel cell system and ideally upon downtime conditioning taking place later, possibly as a function of the temperature. Ideally in this case the measured values or the classes to which the measured values are assigned can be associated with the different system preparation routines by means of a characteristic field.
Further advantageous embodiments of the method according to the invention are disclosed by the rest of the dependent subordinate claims and are made clear with reference to an embodiment, which is described in greater detail below with reference to the drawings.